The present invention relates to the general field of navigation equipment, and more particularly to inertial units.
More particularly, the invention relates to validating an inertial unit of a moving body on board a movement simulator.
In known manner, an inertial unit is a piece of navigation equipment fitted to a moving body (e.g. an aircraft, a rocket) and including measurement instruments such as accelerometers or gyros (rate gyros or free gyros). An inertial unit makes use of measurements performed by such instruments to deliver inertial information to the computer on board the moving body, which inertial information relates, for example, to the angular speed and to the acceleration of the moving body.
On the basis of this inertial data, the computer on board the moving body estimates the three-dimensional position of the moving body and, as a function thereof, it delivers orders or commands to piloting members of the moving body (e.g. airfoil control surfaces, valves, etc.) in order to direct the moving body towards an intended destination and along a determined trajectory.
If the inertial unit presents a fault, the on-board computer estimates the three-dimensional position of the moving body on the basis of inertial information that is inexact. It therefore makes errors at each calculation step and sends erroneous commands to the piloting members. Consequently, the true trajectory of the moving body is very different from the trajectory as estimated by the on-board computer. Thus, if the inertial unit presents a severe defect, the commands sent by the on-board computer are completely inappropriate and the moving body runs the risk of being destabilized.
In order to monitor and validate the functioning of inertial units, use is often made of angular movement simulators that are implemented in hybrid simulations. Such hybrid simulations make use firstly of real subassemblies (e.g. an inertial unit, an angular movement simulator, the on-board computer of the moving body, certain other components of the moving body, etc.) and also of mathematical models of other subassemblies (e.g. concerning propulsion or elements of the environment, such as a mathematical model of the atmosphere, of the earth, etc.).
The angular movement simulators implemented in such hybrid simulations serve to reproduce the angular movements of the moving body in terms of angular range, speed, and acceleration, but they do not enable movements in translation to be reproduced. Consequently, the information from the accelerometers of the inertial unit as supplied to the on-board computer is incomplete since it does not include any information relating to linear movement of the moving body.
In order to mitigate that problem, there presently exist two hybrid simulation strategies that differ in terms of the accelerometer information that is taken into account.
The first strategy consists in replacing the information from the accelerometers of the inertial unit with accelerometer information coming from a mathematical model. In that first strategy, any anomalies that might be present in the accelerometer information delivered by the inertial unit cannot be detected by the hybrid simulation since there is no contribution from the accelerometers of the inertial unit.
The second strategy consists in taking the information from the accelerometers of the inertial unit and adding thereto information that is representative of movements in translation, as calculated using a mathematical model. However, in that second strategy, the information coming from the accelerometers of the inertial unit is measured at a fixed point corresponding to the coordinates of the simulation laboratory. That information is therefore not entirely representative of the information that the accelerometers would supply on the basis of the same physical origins while actually following the trajectory of the moving body around the terrestrial globe. For example, the gravity acting at the fixed point of the laboratory does not vary, whereas the gravity sensed by the accelerometers of an inertial unit on board a moving body that is moving around the terrestrial globe varies as a function of altitude and as a function of latitude. This difference thus falsifies the trajectory calculation of the moving body as obtained by using the inertial unit at the fixed point of the laboratory, and makes it difficult to interpret the results. As a result, that second strategy is capable of detecting only coarse defects in any of the accelerometers of the inertial unit.
Similarly, the information from the gyros of the inertial unit is likewise not necessarily representative of what it would have been were the moving body moving around the terrestrial globe. The rotation of the earth as resolved onto the gyro axes differs depending on whether the inertial unit is situated at a point having fixed coordinates (as applies to hybrid simulation in a laboratory) or whether it is on board a moving body that is moving around the terrestrial globe. The effect of this incomplete representativity is such as to complicate the analysis of the results obtained when performing hybrid simulation. Thus, a trajectory may be obtained for the moving body that is different from the expected trajectory, but without it being certain whether or not this difference is associated with the incomplete representativity of the gyro information. It is therefore not possible to assert in certain manner and without performing more complete analysis whether or not an inertial unit complies with acceptable tolerances relative to the nominal values specified by the manufacturer of the inertial unit.